1. Field of the Invention
The present invention generally relates to using a family of compounds to adjust the allosteric equilibrium of hemoglobin toward a low oxygen affinity state. Moreover, the invention is related to using the family of compounds for use in diseases with oxygen deficiency and to restore oxygen affinity of stored blood.
2. Description of the Prior Art
Hemoglobin is a tetrameric protein which delivers oxygen via an allosteric mechanism. Oxygen binds to the four hemes of the hemoglobin molecule. Each heme contains porphyrin and iron in the ferrous state. The ferrous iron-oxygen bond is readily reversible. Binding of the first oxygen to a heme requires much greater energy than the second oxygen molecule, binding the third oxygen requires even less energy, and the fourth oxygen requires the lowest energy for binding. Hemoglobin has two .alpha. and two .beta. subunits arranged with a two fold symmetry. The .alpha. and .beta. dimers rotate during oxygen release to open a large central water cavity. The allosteric transition that involves the movement of the alphabeta dimer takes place between the binding of the third and fourth oxygen. The .alpha..sub.1 .beta..sub.1 interface binding is tighter than the .alpha..sub.1 .alpha..sub.2 or .alpha..sub.1 .beta..sub.2 interfaces.
In blood, hemoglobin is in equilibrium between two allosteric structures. In the "T" (for tense) state, hemoglobin is deoxygenated. In the "R" (for relaxed) state, hemoglobin is oxygenated. An oxygen equilibrium curve can be scanned, using well known equipment such as the AMINCO.TM. HEM-O-SCAN, to observe the affinity and degree of cooperativity (allosteric action) of hemoglobin. In the scan, the Y-axis plots the percent of hemoglobin oxygenation and the X-axis plots the partial pressure of oxygen in millimeters of mercury (mm Hg). If a horizontal line is drawn from the 50% oxygen saturation point to the scanned curve and a vertical line is drawn from the intersection point of the horizontal line with the curve to the partial pressure X-axis, a value commonly known as the P.sub.50 is determined (i.e., this is the pressure in mm Hg when the scanned hemoglobin sample is 50% saturated with oxygen). Under physiological conditions (i.e., 37.degree. C, pH=7.4, and partial carbon dioxide pressure of 40 mm Hg), the P.sub.50 value for normal adult hemoglobin (HbA) is around 26.5 mm Hg. If a lower than normal P.sub.50 value is obtained for the hemoglobin under test, the scanned curve is considered to be "left-shifted" and the presence of high affinity hemoglobin is indicated. Conversely, if a higher than normal P.sub.50 value is obtained for the hemoglobin under test, the scanned curve is considered to be "right-shifted" and the presence of low affinity hemoglobin is indicated.
It has been proposed that influencing the allosteric equilibrium of hemoglobin is a viable avenue of attack for treating diseases. The conversion of hemoglobin to a high affinity state is generally regarded to be beneficial in resolving problems with deoxy Hemoglobin-S (sickle cell anemia). The conversion of hemoglobin to a low affinity state is believed to have general utility in a variety of disease states where tissues suffer from low oxygen tension, such as ischemia and radio sensitization of tumors. Several synthetic compounds have been identified which have utility in the allosteric regulation of hemoglobin and other proteins. For example, several new compounds and methods for treating sickle cell anemia which involve the allosteric regulation of hemoglobin are reported in U.S. Pat. No. 4,699,926 to Abraham et al., U.S. Pat. No. 4,731,381 to Abraham et al., U.S. Pat. No. 4,731,473 to Abraham et al., U.S. Pat. No. 4,751,244 to Abraham et al., and U.S. Pat. No. 4,887,995 to Abraham et al. Furthermore, in both Perutz, "Mechanisms of Cooperativity and Allosteric Regulation in Proteins", Quarterly Reviews of Biophysics 22, 2 (1989), pp. 163-164, and Lalezari et al., "LR16, a compound with potent effects on the oxygen affinity of hemoglobin, on blood cholesterol, and on low density lipoprotein", Proc. Natl. Acad. Sci., USA 85 (1988), pp. 6117-6121, compounds which are effective allosteric hemoglobin modifiers are discussed. In addition, Perutz et al. has shown that a known antihyperlipoproteinemia drug, bezafibrate, is capable of lowering the affinity of hemoglobin for oxygen (see, "Bezafibrate lowers oxygen affinity of hemoglobin", Lancet 1983, 881.
German Patent Applications 2,149,070 and 2,432,560, both to Witte et al., disclose compounds which are structurally similar to some of the compounds in the family of compounds defined by this invention. However, the Witte et al. patent applications contemplate use of the compounds for the reduction of serum lipid levels. The Witte et al. patent applications do not provide any indication of the potential use of the compounds for allosteric hemoglobin modification.